Rainbow Electronics MAX1870A - Manual (Page 18)

MAX1870A
Step-Up/Step-Down
Li+ Battery Charger
18 ______________________________________________________________________________________
The input source current is the sum of the MAX1870A
quiescent current, the charger input current, and the
system load current. The MAX1870A’s 6mA maximum
quiescent current is minimal compared to the charge
and load currents. The actual wall adapter current is
determined as follows:
where η is the efficiency of the DC-DC converter (85%
to 95% typ), I
SYS_LOAD is the system load current,
I ADAPTER is the adapter current, and I CHARGE is the
charge current.
By controlling the input current, the current require-
ments of the AC wall adapter are reduced, minimizing
system size and cost. Since charge current is reduced
to control input current, priority is given to system loads.
An internal amplifier compares the sum of (V
CSSP -
V CSSN ) and (V CSSP - V CSSS ) to a scaled voltage set by
the CLS input. Drive V CLS directly or set with a resistive
voltage-divider between REF and GND. Connect CLS
to REF for the maximum input current limit of 105mV.
Sense resistors RS1a and RS1b set the maximum-
allowable wall adapter current. Use the same values for
RS1a, RS1b, and RS2. Calculate the maximum wall
adapter current as follows:
where V
CSST
is the full-scale source current-sense volt-
age threshold, and is 105mV (typ). The internal error
amplifier (GMS) maintains input-current regulation (see
Figure 3 for the Functional Diagram). Typically, connect
a 0.01µF capacitor from CCS to GND to compensate
the source current loop (GMS). See the Charge-Current
and Wall-Adapter-Current Loop Compensation for more
information.
Input Current Measurement
The MAX1870A includes an input-current monitor out-
put, IINP. IINP is a scaled-down replica of the system
load current plus the input-referred charge current. The
output voltage range for IINP is 0 to 3.5V. The voltage
of IINP is proportional to the output current by the fol-
lowing equation:
V IINP = I ADAPTER x RS1_ x G IINP x R7
where I
ADAPTER
is the DC current supplied by the
AC adapter, G IINP is the transconductance of IINP
(2.8µA/mV typ), and R7 is the resistor connected
between IINP and ground.
In the Typical Application Circuit, the duty cycle and
AC load current affect the accuracy of V
IINP (see the
Typical Operating Characteristics).
LDO Regulator
LDO provides a 5.4V supply derived from DCIN. The
low-side MOSFET driver is powered by DLOV, which
must be connected to LDO as shown in Figure 1. LDO
also supplies the 4.096V reference (REF) and most of
the internal control circuitry. Bypass LDO to GND with a
1µF or greater ceramic capacitor. Bypass DLOV to
PGND with a 1µF or greater ceramic capacitor.
AC Adapter Detection
The MAX1870A includes a logic output, ASNS, which
indicates AC adapter presence. When the system load
draws more than 1.5A (for 30mΩ sense resistors and
R7 is 10kΩ), the ASNS logic output pulls high.
Shutdown
When the AC adapter is removed, the MAX1870A shuts
down to a low-power state, and typically consumes less
than 1µA from the battery through the combined load of
the CSIP, CSIN, BLKP, and BATT inputs. The charger
enters this low-power state when DCIN falls below the
undervoltage-lockout (UVLO) threshold of 7.5V.
Alternatively, drive SHDN below 23.5% of V
REFIN or
drive ICTL below V REFIN / 100 to inhibit charge. This
suspends switching and pulls CCI, CCS, and CCV to
ground. The LDO, input current monitor, and control
logic all remain active in this state.
Step-Up/Step-Down
DC-DC Controller
The MAX1870A is a step-up/step-down DC-DC con-
troller. The MAX1870A controls a low-side n-channel
MOSFET and a high-side p-channel MOSFET to a con-
stant output voltage with input voltage variation above,
near, and below the output. The MAX1870A implements
a patented control scheme that delivers higher efficien-
cy with smaller components and less output ripple when
compared with other step-up/step-down control algo-
rithms. This occurs because the MAX1870A operates
with lower inductor currents, as shown in Figure 4.
The MAX1870A proprietary algorithm offers the follow-
ing benefits:
• Inductor current requirements are minimized.
• Low inductor-saturation current requirements allow
the use of physically smaller inductors.
• Low inductor current improves efficiency by reducing
I 2 R losses in the MOSFETs, inductor, and sense
resistors.
I
V
V
x
V
RS
ADAPTER MAX
CLS
REF
CSST
_
_
=
1
I I
I x V
V x
ADAPTER SYS LOAD
CHARGE BATT
IN
= +
_
η